The role of the IPCC is to assess on a comprehensive, objective, open and transparent basis the scientific, technical and socio-economic information relevant to understanding the risk of human-induced climate change, its potential impacts and options for adaptation and mitigation.

The IPCC have issued three Summary Assessment Reports; the first in 1990 and the second in 1995 that provided key input to the negotiations leading to the adoption of the Kyoto Protocol in 1997.

The Third Assessment Report was completed in 2001, from which the text on this page has been taken. With another six years of continued observation and research and greatly improved computing power and climate models, the Third Assessment Report makes pretty convincing reading that Climate Change is a reality.

Now that the Summary of the Fourth Assessment Report has been released, it's pretty clear that we're already heading for trouble. We'd better get some changes happening..

Climate Changes so far

The global average surface temperature (the average of near surface air temperature over land, and sea surface temperature) has increased since 1861. Over the 20th century the increase has been 0.6 ± 0.2°C. In the mid- and high latitudes of the Northern Hemisphere over the latter half of the 20th century, it is likely that there has been a 2 to 4% increase in the frequency of heavy precipitation events.

On average, between 1950 and 1993, night-time daily minimum air temperatures over land increased by about 0.2°C per decade. This has lengthened the freeze-free season in many mid- and high latitude regions. There has been a widespread retreat of mountain glaciers in non-polar regions during the 20th century. Northern Hemisphere spring and summer sea-ice extent has decreased by about 10 to 15% since the 1950s.

Tide gauge data show that global average sea level rose between 0.1 and 0.2 metres during the 20th century. Global ocean heat content has increased since the late 1950s.

Global Gases

The atmospheric concentration of carbon dioxide (CO2) has increased by 31% since 1750. The present CO2 concentration has not been exceeded during the past 420,000 years and likely not during the past 20 million years. The current rate of increase is unprecedented during at least the past 20,000 years.

About three-quarters of the anthropogenic (caused by humans) emissions of CO2 to the atmosphere during the past 20 years is due to fossil fuel burning. The rest is predominantly due to land-use change, especially deforestation.

The atmospheric concentration of methane (CH4) has increased by 1060 ppb (151%) since 1750 and continues to increase. The present CH4 concentration has not been exceeded during the past 420,000 years.

Natural Climate Factors

Natural factors have made small contributions to radiative forcing over the past century. The combined change in radiative forcing of the two major natural factors (solar variation and volcanic aerosols) is estimated to be negative for the past two, and possibly the past four, decades. In other words, variability in solar irradiance and volcanic eruptions do not explain the warming in the second half of the 20th century.

Climate Models are Getting Better

Understanding of climate processes and their incorporation in climate models have improved, including water vapour, sea-ice dynamics, and ocean heat transport.

A climate model can be used to simulate the temperature changes that occur both from natural and anthropogenic (human) causes. The simulations represented by the band in (a) were done with only natural forcings: solar variation and volcanic activity. Those encompassed by the band in (b) were done with anthropogenic forcings: greenhouse gases and an estimate of sulphate aerosols, and those encompassed by the band in (c) were done with both natural and anthropogenic forcings included. From (b), it can be seen that inclusion of anthropogenic forcings provides a plausible explanation for a substantial part of the observed temperature changes over the past century, but the best match with observations is obtained in (c) when both natural and anthropogenic factors are included. These results show that the forcings included are sufficient to explain the observed changes, but do not exclude the possibility that other forcings may also have contributed. Similar results to those in (b) are obtained with other models with anthropogenic forcing.

Simulations of the response to natural forcings alone (i.e., the response to variability in solar irradiance and volcanic eruptions) do not explain the warming in the second half of the 20th century (see for example Figure 4a). However, they indicate that natural forcings may have contributed to the observed warming in the first half of the 20th century.

The Findings

The Second Assessment Report concluded: The balance of evidence suggests a discernible human influence on global climate. Since then, progress has been made in reducing uncertainty, particularly with respect to distinguishing and quantifying the magnitude of responses to different external influences. Although many of the sources of uncertainty identified in the SAR still remain to some degree, new evidence and improved understanding support an updated conclusion.

There is a longer and more closely scrutinised temperature record and new model estimates of variability. The warming over the past 100 years is very unlikely to be due to internal variability alone, as estimated by current models. Reconstructions of climate data for the past 1,000 years also indicate that this warming was unusual and is unlikely to be entirely natural in origin.

The warming over the last 50 years due to anthropogenic greenhouse gases can be identified despite uncertainties in forcing due to anthropogenic sulphate aerosol and natural factors (volcanoes and solar irradiance). Changes in natural forcing during most of this period are also estimated to be negative and are unlikely to explain the warming.

"There is new and stronger evidence that most of the warming observed over the last 50 years is attributable to human activities."

Furthermore, it is very likely that the 20th century warming has contributed significantly to the observed sea level rise, through thermal expansion of sea water and widespread loss of land ice.

As Henry Porter observed at the Climate Change Conference in 2005, "Even the scientists were shocked by how advanced various manifestations of global warming were."

Dangerous climate change is on our doorstep. When will we acknowledge the experts?